This invention involves an improved process for the preparation of organophosphorus compounds, e.g., tertiary phosphine oxides and phosphonate esters, such as dialkyl N,N-dialkylcarbamoylmethylphosphonates, which are useful as reagents for the fractionation of radionuclides (e.g., transplutonium elements) from nuclear process streams by solvent extraction (e.g., see U.S. Pat. No. 3,993,728 to Schultz) and/or for extracting uranium from wet process phosphoric acid.
One prior art process, based on the Arbuzov rearrangement, typically affords impure products in about 40% yield (see Siddall III, T. H., J. Inorg. Nucl. Chem., 25, 883 (1963), while another, based on the Michaelis-Becker reaction, produces a similar product in about 40-60% yield (Ibid., 26, 1991 (1964)). The side reactions typical of these prior art methods are described by Petrov, et al, J. Gen. Chem. U.S.S.R., 30, 1604 (1960), and are detrimental to the yields and to the product purities desired for these high boiling liquids. The present invention can allow high yield and purity, for example, of dialkyl phosphonates and trialkylphosphine oxides via a Michaelis-Becker reaction facilitated by liquid-liquid phase transfer catalysis (sometimes herein after called PTC). Phase transfer catalysis has been described, for example, by W. P. Weber and G. W. Gokel, Phase Transfer Catalysis in Organic Synthesis, Springer-Verlag, New York (1977), and by C. M. Starks and C. Liotta, Phase Transfer Catalysis: Principles and Techniques, Academic Press, New York (1978).
Hereinafter, sometimes, the following general equations and formulae will be referred to by the indicated number. ##STR1##
Siddall disclosed the reaction of trialkyl phosphites, of formula, 1, with N,N-dialkylchloroacetamides of formula, 2, (Eq. 1), to be sluggish, requiring forcing conditions (e.g., high temperatures). The reactivity of compounds of formula 1 is further reduced when R is large, as in the case of the preparation of solvent extraction reagents where hydrophobicity is required.
Although the nucleophilic displacement of compounds of formula 2 by alkali metal salts of dialkyl phosphites, of formula 4 (Eq. 2), occurs under considerably milder conditions, the necessary use of strong anhydrous bases, such as sodium metal (to generate the alkali metal salts), is inconvenient on a large scale and leads to side reactions of a magnitude similar to the former route.
M. Fedorynski, K. Wojciechowski, Z. Matacz, and M. Makosza, J. Org. Chem., 43, 4682 (1978), and Polish Patent 105,428 (1980), Chemical Abstracts, 93:953994 reported that a reactive dialkyl phosphite, of formula 4 where R is C.sub.2 H.sub.5, reacts with active alkylating agent benzyl chloride in the presence of potassium carbonate and tetrabutylammonium bromide at 100.degree. C. to afford diethyl benzylphosphonate in 66% yield. It is likely that this reaction proceeds by proton abstraction on the surface of the solid carbonate, providing a means of alkylating reactive, but hydrolytically unstable dialkyl phosphites.
G. M. Kosolapoff and L. Maier, Organic Phosphorus Compounds, Vol. 5, Wiley Interscience, New York, pp 41-2 (1973), note that it is generally known that dialkyl phosphites are vulnerable to hydrolysis in aqueous environments, either acidic or basic.